The present invention relates generally to apparatus for modulating a coherent optical wave and more particularly to an apparatus for modulating a coherent optical wave in response to plural electric signals capacitively coupled to plural portions of an optical waveguide arrangement responsive to the coherent optical wave.
FIGS. 1 and 2 are respectively a schematic diagram and a cross-sectional view of an apparatus for imposing amplitude modulation on an amplitude modulated coherent wave optical beam laser 10 derives. The amplitude modulation of the beam laser 10 emits is in response to an AC, usually RF, signal that source 12 derives and applies directly to laser 10. Optical modulator 16 heterodynes the amplitude modulated beam laser 10 derives with the output signal of RF source 14. Modulator 16 thus derives a coherent wave optical beam having amplitude variations directly proportional to the product of the signals RF sources 12 and 14 derive. Under idealized circumstances, optical modulator 16 derives a coherent wave optical beam having components that are directly proportional to:
AB sin[(xcfx891xe2x88x92xcfx892)t+(xc3x81xe2x88x92xc3x82)]xe2x80x83xe2x80x83(1)
AB sin[(xcfx891xe2x88x92xcfx892)t+(xc3x81+xc3x82)]xe2x80x83xe2x80x83(2),
where
A and B are respectively the peak amplitudes of the signals sources 12 and 14 derive,
xcfx891 and xcfx892 are respectively the angular frequencies of signals that sources 12 and 14 derive, and
xc3x81 and xc3x8 are respectively the phase angles of the signals that sources 12 and 14 derive.
The coherent wave optical beam optical modulator 16 derives also frequently includes components that are directly proportional to:
A sin(xcfx891t+xc3x81)xe2x80x83xe2x80x83(3)
B sin(xcfx892t+xc3x82)xe2x80x83xe2x80x83(4).
Optical modulator 16 includes fiber optic waveguide 18 embedded in solid dielectric plate 20, so that input face 22 of fiber optic waveguide 18 is positioned to be responsive to the coherent wave optical beam laser 10 derives. Modulator 16 includes metal electrodes 24 and 26, plates coated on the top surface of dielectric plate 20 on opposite sides of fiber optic waveguide 18. RF source 14 drives electrodes 24 and 26 by virtue of an ungrounded output terminal of source 14 being connected to electrode 24 and a grounded output terminal of source 14 being connected to grounded electrode 26. Electrodes 24 and 26 are capacitively coupled to fiber optic waveguide 18 so that electric field 19 (FIG. 2), established between electrodes 24 and 26, is coupled to the portion of waveguide 18 between electrodes 24 and 26. As illustrated in FIG. 2, electric field 19 penetrates through solid dielectric plate 20 as well as the portion of fiber optic waveguide 18 between electrodes 24 and 26.
The electric field variations that RF source 14 establishes in the portion of fiber optic waveguide 18 between electrodes 24 and 26 amplitude modulates the coherent wave optical beam laser 10 derives. The resulting coherent wave optical beam in the portion of fiber optic waveguide 18 downstream of electrodes 24 and 26 thus includes the frequency components of RF sources 12 and 14, as sum and difference frequencies that are amplitude modulated on the optical carrier frequency of laser 10. The amplitude of the coherent optical wave downstream of electrodes 24 and 26 can thus be considered as the product of the output signals of RF sources 12 and 14. The portion of optical fiber waveguide 18 downstream of electrodes 24 and 26 supplies the coherent wave optical beam including the products resulting from multiplication of the signals of sources 12 and 14 to a suitable optical-electric transducer 28. Transducer 28, typically a photo-electric detector, such as a diode or transistor, derives an electric signal that is a replica of the amplitude variations of the coherent wave optical beam incident on it, i.e., the optical beam at the output of modulator 16.
A problem with the structure illustrated in FIG. 1 is that the transfer function of optical modulator 16 in response to the signal that source 14 applies to electrodes 24 and 26 is quite different from the transfer function of laser 10 in response to RF source 12. These transfer function differences are such that the response time of laser 10 to RF source 12 is considerably different from the response time of modulator 16 to RF source 14. In addition, laser 10 and modulator 16 have different non-linearities. Calibrating the apparatus illustrated in FIG. 1 is difficult because of these factors.
It is, accordingly, an object of the present invention to provide a new and improved apparatus for modifying a coherent optical wave in response to at least two electric signals that act on the wave in substantially the same way.
Another object of the invention is to provide a new and improved apparatus for amplitude modulating a coherent optical wave in response to two or more electric signals that are coupled to the coherent beam with substantially the same transformer function and by the same mechanism.
In accordance with one aspect of the invention, an apparatus for modulating a coherent constant amplitude optical wave in response to at least first and second AC electric signal sources comprises an optical waveguide arrangement arranged to be responsive to the optical wave. A first pair of electrodes connected to be responsive to the first AC source and capacitively coupled to a first portion of the optical waveguide arrangement modulates the optical wave propagating in the first portion of the optical waveguide arrangement in accordance with the first AC source. A second pair of electrodes connected to be responsive to the second AC source and capacitively coupled to a second portion of the optical waveguide arrangement modulates the optical wave propagating in the second portion of the optical waveguide arrangement in accordance with the second AC source. The first and second portions of the optical waveguide arrangement are coupled together so that the modulated coherent optical wave derived by the first portion and the modulated coherent optical wave derived by the second portion are combined to derive a third modulated coherent optical wave.
In one embodiment, the second portion of the optical waveguide arrangement is cascaded with first portion of the optical waveguide arrangement so that the second portion of the optical waveguide arrangement derives a coherent optical wave including components containing the sum and difference frequencies of the first and second sources.
In a second embodiment, the optical waveguide arrangement includes a third portion connected to be responsive to the coherent optical waves the first and second portions derive. The first, second and third portions are preferably arranged so the coherent optical waves the first and second portions derive propagate toward each other when entering the third portion. The third portion includes an output optical waveguide segment responsive to the coherent optical waves propagating toward each other from the first and second portions. Preferably, the optical waves the first and second portions derive are supplied to a third portion by aligned optical waveguide segments and via one-way mirrors.
In the preferred arrangement of the second embodiment, the output optical waveguide segment is at an oblique angle to the aligned optical segments, to provide a convenient structure for linearly combining the amplitudes of the optical waves in the aligned optical segments.
Each of the first and second portions of the optical waveguide arrangement preferably includes fiber optic waveguides embedded in a solid dielectric medium and comprises first and second spaced electrodes. The first and second spaced electrodes in both portions are carried by the solid dielectric medium and connected to be responsive to the electric sources.
The above and still further objects, features and advantages of the present invention will become apparent upon consideration of the following detailed description of specific embodiments thereof, especially when taken in conjunction with the accompanying drawings.